Hi I'm 21 yrs. Old , I have regular monthly periods that always happen on the same day every month but this month It came late instead of the 3rd it occurred on the 10th then I had sex with my partner on the 21st which indicated I was on fetile days but yesterday I started spotting & today I woke up nauseous, bad headache, & real severe cramps also I'm having heavy bleeding it's starting to worry me because Ive never had this happen to me please help

Sounds like you're having an off month.